Humans are the only vertebrate hosts. Vibrio cholerae can survive long-term and probably permanently in brackish water, especially if there is a a neutral or slightly alkaline pH and the water contains some minerals and organic material. The bacteria are concentrated in phytoplankton (certain algae) and zooplankton which live in this water. Among the latter, copepods, a group of crustaceans, are important. V. cholerae O1 produces chitinase, an enzyme which can break down chitin. This is a structural protein of the exoskeletons of invertebrates. The full importance of this chitinase has still to become clear.

Vibrio cholerae is also found in association with a cyanobacterium (Anabaena variabilis). Vibrio cholerae has similar interactions with marine diatoms (Skeletonema costatum), phyaeophytes (Ascophyllum nodosum) and copepods. The bacteria also flourish in fresh water where plants such as water hyacinth are growing. The impact of specific ecological circumstances which might lead to a sudden increase in plankton (algal bloom) needs to be studied further. Cholera is clearly seasonal. A chronic aquatic reservoir is likely and this might be independent of continuous human faecal pollution. V. cholerae excreted by humans can be cultured in the laboratory. These bacteria may assume a living form which cannot be cultured in vitro and which multiply in the environment. However, those bacteria are not dead since they multiply when instilled in a rabbit's ileum (rabbit ileal loop model). It may revert to a replicating form in its natural environment when there are favourable environmental factors and this has important epidemiological implications. The living, but non-reproducing form of V. cholerae can probably cause disease. Traditional culture methods for tracing V. cholerae in water miss these “dormant” bacteria. Tests based on fluorescent antibodies may offer a practical solution.

Half empty cargo ships take water in their ballast tanks in order to lie deeper and with more stability in sea water. This ballast water of course contains micro-organisms. In the past potentially infected ballast water was pumped out again when the ship came into harbour at its destination. In view of the importance of an aquatic cholera reservoir, freight ships have to empty and refill their ballast water tanks while in open sea, and not when they come into harbour.

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